feat(player): resume last session on launch (#54)

The #52 teardown clears the session when idle/dismissed, so the
headset / lock-screen play button had nothing to resume and the user
lost their place.

- track.dart: toJson() (round-trips fromJson)
- db.dart: CachedResumeState single-row snapshot, schema 9->10 + migration
- audio_handler.dart: queuedTracks getter
- player_provider.dart: restoreQueue() — configure + setQueueFromTracks
  + seek, no play (restores PAUSED)
- resume_controller.dart: restores last snapshot paused on launch;
  persists {source,index,position_ms,tracks} debounced on track
  change / pause and immediately on app teardown; never clobbers the
  saved snapshot when the queue is empty so a teardown stays
  recoverable; restore gated on auth + no active session
- app.dart: wired into postFrame

db.g.dart regenerated by CI per project convention. Deferred fast-follow:
media-button-when-fully-stopped re-init (needs a configure()-injected
callback; tracked on #54).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/models/track.dart

@@ -40,4 +40,19 @@ class TrackRef {
         durationSec: (j['duration_sec'] as num?)?.toInt() ?? 0,
         streamUrl: j['stream_url'] as String? ?? '',
       );
+
+  /// Round-trips through [TrackRef.fromJson] (same server snake_case
+  /// keys). Used to persist the playback queue for resume-on-launch.
+  Map<String, dynamic> toJson() => {
+        'id': id,
+        'title': title,
+        'album_id': albumId,
+        'album_title': albumTitle,
+        'artist_id': artistId,
+        'artist_name': artistName,
+        if (trackNumber != null) 'track_number': trackNumber,
+        if (discNumber != null) 'disc_number': discNumber,
+        'duration_sec': durationSec,
+        'stream_url': streamUrl,
+      };
 }